Problem: The expression $4x$ gives the perimeter of a square with a side length of $x$ units. What is the perimeter of a square with a side length of $\dfrac57$ units?
Solution: A side length of ${\dfrac57}$ units tells us that $x={\dfrac57}$. Let's substitute $x={\dfrac57}$ into the expression and evaluate: $\begin{aligned} &\phantom{=}4x\\\\ &= 4\left({\dfrac57}\right)\\\\ &= {\dfrac{20}7}\\\\ \end{aligned}$ A square with a side length of ${\dfrac57}$ units has a perimeter of ${\dfrac{20}7}$ or ${2\dfrac67}$ units.